STRUCTURAL AND OPTICAL INVESTIGATIONS OF TGB(A) AND TGB(C) MESOPHASESEXHIBITING CYLINDRICAL AND CONE-LIKE DOMAIN TEXTURES

The characterization by optical microscopy, X-ray diffraction and diff erential scanning calorimetry of two new liquid crystalline compounds is presented. The compounds under consideration incorporate in their m olecules two chiral centres (one asymmetric carbon and one asymmetric sulphur of a sulphinate group). While the carbon chirality is fixed in the R configuration the sulphur chirality mag. be either in the S or in the R configuration. A TGB(A) mesophase in a large temperature doma in is evidenced for one of the two diastereomers, while both TGB(A) an d TGB(C) mesophases are observed in a similar temperature domain for t he other one. For both diastereomers and in both mesophases it is poss ible to observe the coexistence of two different types of optical text ures, namely planar cholesteric textures and developable domains. The latter are coiled in a different way than proposed in an earlier publi cation. More precisely, they form cylindrical or cone-like domains. wi th double-twist properties as observed in the blue phases. The pitch o f the helix in the TGB mesophases is evaluated by means of optical ref lection observations in samples exhibiting the planar cholesteric text ures. For both diastereomers it is also possible to evaluate structura l parameters related to the organization within the TGB(A) mesophases. Finally, the induced phase transition TGB(C)-S-C is studied by the a pplication of an AC electric field.